How to Prevent Cross-Site Scripting

When it comes to detecting types of security threats, cross-site scripting (XSS) website vulnerabilities are decades-old and still widely used by attackers today.


Cross-site scripting (XSS) attacks have been listed in the OWASP Foundation’s Top 10 vulnerabilities since day one and is currently the third most prominent vulnerability (as part of the Injection category).

While cross-site scripting attacks may be old school, the potential damage they can cause is significant. Victims of an XSS attack may be vulnerable to financial fraud, data theft, or identity theft.

Knowing if you’re vulnerable and how to prevent cross-site scripting from happening is critical; fortunately, cross site scripting prevention methods is not all that difficult.

What is XSS

What is cross-site scripting?

  • Cross-site scripting (XSS) is a type of malicious attack that tricks a user into executing malicious code on the victim's browser in the context of the user's current web page. XSS attacks can take many forms, including embedding malicious scripts within a web page or injecting them into an HTTP request.


    Once executed, XSS code can access and manipulate any information accessible via the victim's browser — including cookies, login credentials, and more.


    The most common XSS vulnerabilities exist in programming languages like JavaScript, ActiveX, Flash, and VBScript. Because it’s so closely integrated with most browsers, JavaScript is the most prominent.

How XSS works

How do XSS attacks work?

  • The better question here is probably why cross-site scripting works.


    It all boils down to a case of too much trust.


    By default, web browsers behave as if they trust all the code corresponding to the websites they load. Browsers are not expecting abnormal and insecure code, so they will process it as if it’s normal when they see come across it. 


    With many current XSS attacks, an attacker can add malicious code to the end of the URL or post code onto a webpage displaying user-generated data.


    XSS vulnerabilities are widespread, and they’re an easy way for attackers to get what they want. If your web application is not set up to validate or encode user input, the browser can be vulnerable because it will trust — and therefore process — the malicious code.


    The degree of success for attackers is typically high.

Effects of XSS

What can happen with an XSS attack?

  • With an XSS attack, the attacker is looking to gain access to your resources or data. With a successful attack, the adversary might be able to read and modify confidential data, impersonate your employees, and make changes to your web pages or applications.


    Once you’re compromised, the attacker can capture keystrokes, steal access cookies and session tokens, or access sensitive browser data. Armed with this treasure, they can begin sending messages or conducting financial transactions.

Find vulnerabilities

How to find vulnerabilities in a website

  • To discover your vulnerability to cross-site scripting attacks, the simplest methods are performing manual penetration testing or using a vulnerability scanner.


    But it’s important to note that some XSS flaws can be difficult to identify (and subsequently remove). Experts from the OWASP foundation suggest performing a security review of the code and searching for all places where input from an HTTP request could possibly make its way into the HTML output.


    OWASP maintains a comprehensive list of free and paid online XSS vulnerability scanners you can use to scan your website for flaws. However, these may only scratch the surface — as they only shine the light on one specific vulnerability.

XSS defenses

What are the different cross site scripting defenses?

  • Defending against a cross-site scripting attack or XSS vulnerability can’t be approached with a simple fix that can be applied quickly. Different applications require specific XSS protection strategies.


    Promote security awareness

    To ensure your web applications are secure, all stakeholders must be aware of the potential security risks that attacks such as XSS pose. Providing security training for your developers, DevOps staff, QA employees and SysAdmins is crucial.


    Validate inputs

    If your web application accepts form inputs, you’ll want to validate the data to ensure it meets whatever criteria you deem normal or secure. In simple terms, validation means to either accept or reject the input based on its content.

  • Sanitize your data

    The concept of sanitization is too complex for the purposes of this article, but what you need to know is that sanitizing essentially cleans up the data by removing abnormal, irrelevant or dangerous characters. An online tool like HTMLSanitizer can sanitize HTML code for XSS vulnerabilities.


    Consider prohibiting HTML code in inputs

    If users can’t post HTML code into form inputs, they can’t execute an attack through that avenue.


    Keep your cookies safe  

    With web application rules that specify how cookies are handled, the probability of an XSS attack diminishes.


    Use a Web Application Firewall - A web application firewall (WAF) can define rules that  specifically apply to XSS, preventing any abnormal server requests. A key benefit of leveraging a  robust WAF security as a part of your company’s security strategy is that it also helps prevent attacks like DDoS, SQL injection and other common threats.

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